1. Technical Field
The present disclosure relates to a toner and a method of manufacturing the toner.
2. Discussion of the Background
Currently, there is a tendency of supersession from monochrome to full-color printing with regard to a photocopier, a printer, and a multi-function printer/peripheral/product (MFP), which functions as one apparatus including the function of a photocopier and a printer using electrophotography.
Among them, MFPs and printers installed in a small-sized office or distributed in an office have a large volume of shipments and are desired to be compact in size and inexpensive.
To satisfy these needs, the non-magnetic single component development process has an advantage in terms of the number of parts required. Further, an oil-free fixing process is adopted to dispense with an oil application mechanism and furthermore, it is preferred to adopt a cleaner-less system which can exclude a cleaning mechanism for an image bearing member, for example, a photoreceptor, and an intermediate transfer belt, if desired.
The non-magnetic single component development process is a method by which toner particles are frictionally charged by a toner layer regulating member contacting with a toner bearing member, thinly coated on the toner bearing member at the same time and transferred to the developing area where the toner bearing member opposes a latent electrostatic image bearing member to develop the latent electrostatic image on the latent image bearing member and visualize the image as a toner image.
In the non-magnetic single component development, toner is charged by friction between the toner and a toner layer regulating member. The toner layer regulating member contacts with the toner bearing member with a pressure in some degree to uniformly impart sufficient charge thereto. When development is continuously performed, problems arise such that toner cracks due to the pressure of a toner layer regulating member and toner and/or wax contained therein are fused and/or agglomerated and fixated on the surface of a toner bearing member, resulting in noise, for example, streak, and uneven density on a formed image.
As binder resins for use in toner, polyester resins, which are strong for mechanical stress caused by process, are advantageous and preferably used over an inexpensive styrene-acryl resins in terms of fixing property based on sharp melting property.
Toner used to be manufactured by the following method, which is to: mix a binder resin, for example, a polyester resin, with a coloring agent, wax and optional low molecular weight compounds or resins, for example, a charge controlling agent, a wax dispersing agent and a coloring agent dispersing agent, with a mixer, for example, HENSCHEL MIXER; melt, mix and knead the resultant with an two-axis extruder; pulverize the resultant at several stages to obtain color resin powder having a size of from several μm to several tens of μm; and adjust the size distribution thereof by air-classification. These are referred to as pulverized toner. However, pulverized toner is manufactured with an extremely large amount of energy in the pulverization process. To manufacture toner having a small particle diameter to restrain the consumption amount thereof and to improve the quality of images, this energy increases, which relates to a problem of burden on environment. With regard to quality, since it is difficult to control the dispersion of each material contained in toner, a fatal problem that wax exposes to the surface of a toner particle inevitably occurs. Therefore, pulverized toner has difficulty as toner for use in the oil-free fixing process.
To solve these problems, toner manufactured by various kinds of methods have been marketed in recent years instead of the pulverized toner.
For example, unexamined published Japanese patent applications Nos. (hereinafter referred to as JOP) H11-143125 and 2002-148858 describe a method for obtaining toner particles having a uniform shape, a sharp size distribution and a sharp charge amount distribution. There is, for example, a method of association (agglomeration and/or fusion) of resin particles obtained by emulsion polymerization method with coloring agent particles and wax particles. In this method, it is possible to obtain toner particles having a significantly spherical form. However, due to fragility of the styrene-acryl resin obtained by emulsion polymerization, toner particles tend to crack at the cohesion interface thereof by the pressure of a toner regulating member, resulting in deterioration of image quality.
Therefore, JOPs H11-7156 and 2004-271808 describe a method in which aqueous dispersion body of polyester particulate containing wax and a coloring agent is agglomerated and/or fused to obtain toner.
JOPs 2002-82485 and 2004-287149 describe a method in which polyester particulates, wax particulates and coloring agent particulates are agglomerated and/or fused in an aqueous medium to obtain toner.
JOPs 2006-91564 and 2006-91379 describe a method in which crystalline polyester particulates, wax particulates and coloring agent particulates are agglomerated in an aqueous medium, and non-crystalline polymer particulates are agglomerated to the agglomerated body to obtain toner particles.
The toners obtained by these methods are excellent in fixing property. However, it is found that toner cracking and contamination on peripheral devices occur while mixing and regulation are repeated in a developing device. This is probably because the wax and the polyester resin are not sufficiently adhered to each other and the toner cracks at the interface therebetween by the mechanical stress.
JOP 2004-295105 describes the following method: an oil phase in which a polyester resin, a styrene-acryl resin and wax are dissolved or dispersed in an organic solvent is prepared and poured in an aqueous phase containing a dispersing agent followed by stirring to finely-disperse the oil phase; and the organic solvent is removed to obtain liquid dispersion of resin containing wax followed by agglomeration and/or fusion. The resin particulates obtained here is desired to contain wax therein. However, wax is locally existent on the surface of a particle since the wax moves together with the solvent when the solvent is removed. Furthermore, phase separation of the polyester and the styrene-acryl occurs when the solvent is removed. Therefore, when this method is applied, the phase separation inside a toner particle causes deterioration of toner strength so that it is difficult to sufficiently prevent toner cracking.
Japanese patent No. 3456372 describes the following method: an oil phase in which polyester, a coloring agent and wax are dissolved or dispersed in an organic solvent is prepared and poured into an aqueous phase containing a dispersing agent followed by relatively gentle stirring to disperse the oil phase; and the organic solvent and the dispersing agent are removed followed by washing, drying and classifying to obtain toner. However, even the toner obtained by this method tends to crack or contaminates peripheral members when mixture and regulation are repetitively performed in a developing device. The polyester and the wax have a weak adhesion property at the interface therebetween, which is thought to be a trigger to cause toner particles to crack.
JOPS S36-10231, S59-53856 and S59-61842 describe the following method: an oil-soluble polymerization initiator, a coloring agent, wax, a charge controlling agent, etc., are dissolved or dispersed in a vinyl-based monomer; the resultant is suspended in an aqueous medium containing an organic and/or inorganic dispersing agent; and the resultant is heated for polymerization to obtain toner particles. This method is referred to as a suspension polymerization method and the concept thereof has been well known for a long time. However, the fragility of styrene-acryl resin obtained from polymerization of a vinyl-based monomer causes a problem due to the reason described above. It is possible to overcome the fragility by cross-linking a resin but this sacrifices the fixing property.
JOPS S56-116042, S56-116043, and S60-238846 describe a method in which polyester is dissolved or dispersed in a vinyl-based monomer to suspension polymerization toner containing polyester. However, there is a limit to the amount of polyester which can be dissolved in a vinyl based monomer. As described in the specifications of these JOPs, a preferred amount of the polyester is not greater than a half against the amount of vinyl based monomer. The toner obtained by these methods does not exploit the characteristics of polyester resin and cannot restrain the occurrence of toner cracking and contamination to peripheral members.
Consequently, toner having a good combination of the fixing property and durability has not been obtained by these methods described above.
In addition, with regard to electrophotography, various kinds of studies and development have been made with creativity and technology approaches. In electrophotography, a toner image is formed by the following process: charging and irradiating the surface of an image bearing member to form a latent electrostatic image; developing the latent electrostatic image with a color toner to form a toner image; transferring the toner image to a transfer medium (e.g., sheet); and fixing the toner image with a heat roller, etc.
As the method of fixing toner, a contact and heating fixing method, e.g., a heat roll method, has been widely adopted. A fixing device for use in such a heat roll fixing method includes a heat roller and a pressure roller. When a recoding medium bearing a toner image passes through the nipping portion of the heat roller and the pressure roller, the toner image is melted and fixed on the recording medium.
The viscosity of melted toner drastically decreases. Therefore, there is possibility that toner may attach to the heating roller (referred to as offset), a recording sheet is wound round the heating roller and not detached therefrom. To prevent the occurrence of these problems, there is a method of applying a lubricant, for example, silicone oil, to a roller. However, a device to apply a lubricant is required, which increases cost. Also the size of a fixing device increases. In addition, oil is attached to an image, resulting in formation of a glossy image. This makes writing impossible on the image with a pen, etc. To the contrary, there is a method of solving such drawbacks by containing a releasing agent (wax) inside toner particles. In this method, when toner is melted, releasing agent is melted and oozes from the inside of the toner so that attachment of toner to a heating roller can be prevented. The ideal function of such a releasing agent is to: (1) melt at a temperature as low as possible; (2) to melt with a small amount of heat; (3) to have a low fusion viscosity; (4) to securely and quickly move from the inside of a toner particle to the outer surface. When these functions are achieved, the required amount of releasing agent is small, the fixing temperatures can be low, the power consumption of a heating roller, etc., is small, the margin that can be utilized for increasing the system speed (printing speed) can be wide, etc. However, to achieve (1), when the fusion temperature (melting point) of such a releasing agent is too low, a problem may arise that toner containing the releasing agent melts inside an image forming apparatus, for example, a developing device, due to the environmental change and the temperature rise during driving of devices. Namely, there is a limit on simply decreasing the melting point of a releasing agent. In addition, to achieve (2), it greatly depends on the amount of a releasing agent contained in toner. A sufficient releasing effect is not obtained with a small amount of such a releasing agent. To the contrary, when the amount thereof increases, the required amount of heat increases and the releasing agent tends to be present close to or on the surface of toner particles. The condition of (3) depends on the kind of releasing agents. A releasing agent is desired to have a small intermolecular reaction and a low polarity in light of molecular structure. However, when the polarity in a releasing agent changes, the compatibility and interaction between the releasing agent and a binder resin in toner also change. The existing status and position of a releasing agent inside toner should be taken into account. With regard to (4), this highly depends on the existing position and status of a releasing agent inside a toner particle. When a releasing agent is present close to the center of a toner particle, it is difficult for such a releasing agent to quickly move to the surface of the toner particle when heated and melted for fixing. Therefore, it is desired that such a releasing agent is uniformly dispersed or present near the surface of a toner particle in a range in which there is no adverse impact on a developing device, etc. In addition, when the existing status (hereinafter referred to as domain) of a releasing agent is small (fine dispersion), the releasing agent is difficult to quickly move inside a toner particle when heated and melted. Furthermore, the extruding effect by transformation upon application of pressure is difficult to obtain. Especially, a spherical domain is a great disadvantage. Therefore, the domain form is desired to be a stick form or disk form.
As the resin for use in toner, a resin having a vinyl-based polymer resin and a resin having a polyester skeleton are suitably used. These resins have each own merits and demerits on fluidity, movability, chargeability, fixability and image characteristics. In recent years, both resins have been used in combination and/or a resin having both skeletons (referred to as hybrid resin) have been also used. With regard to the method of manufacturing toner, in addition to the typical method, i.e., the kneading and pulverization method, there are methods referred to as wet granulation method or chemical toner method, for example, a suspension method, an emulsifying method, a suspension polymerization method in which toner particles are directly obtained by polymerization while controlling polymerizable monomer droplets, and an agglomeration method in which emulsified particulates are prepared and agglomerated to obtain toner particles.
For example, JOP 2005-084183 describes a toner in which a coating layer formed of resin particles manufactured by an emulsifying polymerization method or an emulsifying dispersion method using a surface active agent covers the surface of a color resin particle manufactured by an emulsifying dispersion method and the core is a polyester-based resin and the coating layer is a vinyl-based resin. JOP 2004-295105 describes a toner manufactured through the process of agglomerating resin particles in an aqueous medium as follows: Prepare liquid dispersion in which a resin solution in which a polyester resin and a styrene-acryl based resin are dissolved in an organic solvent is dispersed in an aqueous medium; Remove the organic solvent from the liquid dispersion; and agglomerate resin particles in the aqueous medium. JOP 2004-271686 describes a toner manufactured as follows: Prepare resin particulates having a size of 1 μm from a polyester-based resin and carnauba wax by polyaddition reaction or polycondensation; Prepare liquid dispersion in which the resin particulates are dispersed in an aqueous medium; and curing salt-out and/or fusing the resin particulates in the liquid dispersion in the aqueous medium. In addition, Japanese Patent No. 3577390 describes a toner manufactured by preparing resin particulates having a size of 0.9 μm from a polyester-based resin and oxidization type polypropylene followed by agglomeration. Furthermore, JOP H11-007156 describes a toner manufactured as follows: Prepare a mixed solution by dissolving or dispersing a toner composition containing a binder resin using multiple polyester resins having different acid values or glass transition temperatures and a coloring agent in an organic solvent; and Introduce the mixed solution into an aqueous medium to manufacture resin particulates having a size of from 0.4 to 0.7 μm from the polyester-based resins and paraffin wax by suspension granulation followed by agglomeration.
These chemical toners have variety of advantages when compared with kneading and pulverization methods. For example, the resin structure can be controlled and the position of a releasing agent, etc., can be also controlled. However, the following is found: When a releasing agent is dispersed in a vinyl-based polymer resin in emulsification, both are compatible with each other, resulting in fine dispersion; In the case of emulsification polymerization, it is difficult to manufacture a large-sized particulate and therefore the domain is not large; With regard to the suspension polymerization, a releasing agent is dissolved in a monomer and a spherical domain is formed in the toner center; and when toner particles are directly granulated by a polyester dissolution suspension method, the releasing agent exposes to the surface of obtained toner particles.
As described above, with regard to the toner prepared by the chemical toner method, toner having excellent fixing properties which can deal with high speed printing as full, color toner while having a satisfactory function and characteristics has not been obtained. There is a need for an improved toner.